142 BIG MOLECULES 



brane because of its unique function in electron transport in the step-wise 

 oxidation of foods. But the general problem of direct knowledge of the struc- 

 ture of living membranes probably awaits more knowledge of the structure of 

 macromolecules in solution. 



Indirect methods — i.e., studies of penetration of the living membrane by 

 water, ions, and molecules — are proving to be very helpful to studies of 

 structure, because from such studies one can infer some properties of the 

 membrane in vivo: pore size, for example. An estimate of pore size (length 

 and area) requires at least two independent experimental measurements, 

 because there are two-dimensional parameters, area and length, to be evalu- 

 ated. Both the rate of diffusion of a substance down a concentration gradi- 

 ent and the rate of flow of a fluid under a mechanical pressure, should be 

 larger the larger the area of the hole in the membrane and the shorter its 

 length. 



Although the rate of transport of water through the cell membrane of 

 erythrocytes is very rapid, both rate of diffusion and rate of flow have re- 

 cently been measured accurately enough to determine a value for average 

 pore diameter in the erythrocyte wall in vivo. Diffusion rate of water was 

 found by measuring the rate at which radioactively labeled water is picked 

 up by the cells within a few milliseconds of being bathed in the labeled 

 water. A fast-flow apparatus had to be used, the ingenious details of which 

 are best described in the original papers. 8 Then the rate of flow into the cell 

 was measured by suddenly changing the osmotic pressure (salt concentra- 

 tion) outside the cell, and following the change in cell diameter by means of 

 a light-scattering technique. 



From the results, an analysis gives about 7 A as the diameter of the pores 

 in the erythrocyte wall. The beauty of this kind of experiment is that it is a 

 measure of a physical property of the membrane while it is living and func- 

 tioning normally. The limitation is that the analysis involves certain as- 

 sumptions which may or may not turn out to be absolutely correct. In the 

 next few years it will be supplemented by the so-called "differential osmotic 

 pressure" approach of Staverman, in which pore size can be inferred by the 

 "ieakiness" of the membrane to certain ions; and by the molecular- or ionic- 

 sieve approach, in which a large number of ions of various sizes are tested for 

 their penetration. The diameter of the largest one which can penetrate the 

 membrane is the effective diameter of the pore. 



Further support for the pore theory comes from examination of mono- 

 molecular layers of large fatty acids and lipids. The lipid is spread out on 

 water in a pan with a moveable boom (the so-called "Langmuir trough"). 

 The boom is then made to reduce the area which the spread lipid must 

 cover, and the force required to move the boom is measured on a sensitive 

 torsion balance. When the layer has closed in completely, the resistance to 



